This invention relates to a parking brake actuator and a parking brake system and, more particularly, to an electrically operated parking brake actuator and system for automotive vehicles.
Electrically operated parking brake actuators have been proposed for setting and releasing the parking brake of an automotive vehicle. These actuators generally include an electric motor connected to a parking brake cable system through a gear train including a sector to which a main brake cable is attached. This main brake cable is connected to a pair of cables each of which is, in turn, connected to a parking brake member. Each parking brake member is associated with a different wheel of the vehicle. Typically the motor and gear train are located in the cockpit of the vehicle although some proposals have located them under the hood adjacent the firewall. To set the parking brake, the motor and, in turn, the gear train are driven in one direction so that the sector tensions the main brake cable which, in turn, tensions the cable pair and applies the parking brake; to release the brake, the motor and gear train are driven in the opposite direction to relieve the tension in the cable system and release the brake.
These prior art systems are too expensive to be widely used. The expense results from a variety of requirements necessitated by the design. A relatively large motor and gear train arrangement are required to provide enough force to tension the cables. The bulk of the gear trains is also utilized to prevent backloading (a force acting to release the brake) the gears and/or motor when the cables are tensioned.
Assembling these systems to a vehicle is also expensive. This expense results from the need to adjust the slack in the cable when the assembly is installed on the vehicle. Usually the slack is removed so that a predetermined minimal tension is left in the cables when the brake is released. Repeatability of the cable tension has also been a problem and often results in a maintenance expense. The predetermined tension relaxes over the life of the vehicle as the cables wear and stretch. Thus adjustment is required to remove the slack after the vehicle has been in use. Some systems address this problem by including an automatic slack adjuster. This additional feature adds to the cost and complexity of the parking brake system.